1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory, specifically to an electrically programmable ROM (Read-Only Memory).
2. Description of the Related Art
In recent years, electronic tags have become widespread use for non-contact object recognition in applications such as material flow control with RF-ID tags, controlled access system and checkout control system. The electronic tags are attached to a large number of objects of recognition, and store digital information on the objects in embedded memories. In general, the digital information mentioned above is programmed and read in a non-contact manner with a reader/writer of a user.
The memory to store the digital information is a mask ROM, or the like. The digital data has been written into the mask ROM and fixed when the mask ROM is completed. Thus, the user of the electronic tag can not program his digital data into the mask ROM, after the electronic tag incorporating the mask ROM is shipped to a market as a product.
Therefore, EPROMs (erasable programmable ROMs) and EEPROMs (electrically erasable programmable ROMs), into which the user can program his data electrically, are often used as the memories incorporated in the electronic tags.
However, a production cost of the electronic tag incorporating the EPROM or the EEPROM of a conventional art is increased because of more complicated manufacturing process due to structures of the EPROM and the EEPROM. There arises a problem that the price of the memory and the price of the electronic tag incorporating the memory are increased.
Furthermore, since the electronic tag is used in various environment such as under ultraviolet light from the sun or at high temperature, it is necessary to apply additional processing in order to adjust the electronic tag to the environment. More specifically, for example, the electronic tag incorporating the EPROM needs a package which shields the EPROM from sunlight including ultraviolet light, since the stored data in the EPROM is erased by ultraviolet light. For that purpose, an additional packaging process is required in manufacturing the electronic tag. In general, the cost of packaging is expensive relative to the manufacturing cost of the semiconductor device itself, thus the cost of packaging significantly increases the manufacturing cost of the electronic tag.
On the other hand, although the user can not program his digital data into the mask ROM of the conventional art, the mask ROM is cheaper than the EPROM or EEPROM, and has high resistance to the environment (hereafter referred to as environmental immunity) such as ultraviolet light and high temperature.
This invention offers a ROM into which digital data can be programmed by the user and suitable as the memory incorporated in the electronic tag described above, or the like.